1. Field of the Invention
The present invention generally relates to the measurement of the thickness of thin films and, more particularly, to a new technique of measuring thin film thickness using ultraviolet fluorescense.
2. Description of the Prior Art
Measurement of a thin film on a thick substrate historically has been a difficult problem. An example of such a measurement is the determination of the amount of adhesive on an envelope flap. The adhesive thickness is on the order of 0.0007 inches thick, while the envelope is on the order of 0.005 inches thick. Because the ratio of adhesive to envelope thickness is about 7.1:1, a 10% change in adhesive thickness presents an overall thickness change of only 1.4%. Thus, measuring thickness of the adhesive layer on an envelope using typical mass or absorption techniques, such as beta ray absorption, is difficult because thickness changes in the adhesive produce only small changes in the total measurement. This measurement problem is common to a large number of situations in which a relatively bulky substrate is coated with a thin film or layer of other material or in which it is desired to measure the thickness of a thin film alone.
U.S. Pat. No. 3,395,278 to McDivitt discloses a method of measuring the coating thickness of articles. In the specific example disclosed, coatings on glass containers are measured using a reflectivity device which employs the use of ultraviolet light. To make the glassware scratch resistant, the glassware is provided with a metallo-organic ester which provides a transparent coating on the glassware. The thickness of the coating is measured by measuring the reflectivity of the coating to the untraviolet light.
Other examples of reflectance type thickness measurement techniques are disclosed in U.S. Pat. No. 3,016,464 to Bailey and U.S. Pat. No. 3,325,649 to Bird. As an alternative to reflection type measurements, it is also possible to measure thickness using an inverse transmission or obscuration type of measurement. Examples of this type of measurement are disclosed, for example, in U.S. Pat. No. 2,813,981 to Friedman and U.S. Pat. No. 3,076,723 to Covington.
U.S. Pat. No. 4,296,326 to Haslop et al. discloses a method of detecting watermarks in paper. This measurement is accomplished by an absorption measurement wherein the reflectance of ultraviolet radiation by a sheet of paper in the vicinity of the watermark is measured. Alternatively, the transmittance or the fluorescence of the sheet of paper in the presence of ultraviolet radiation can be measured. The measured value is compared to a reference value obtained by a measurement of the same parameter outside the watermark area.
U.S. Pat. No. 3,577,885 to Wells discloses a method for detecting a coating transfer from a coated film which involves incorporating a fluorescent material in the coating. The coating is applied to one surface of the film and subsequent scanning of the surface of the film with ultraviolet light will allow detection of the fluorescent material to confirm the successful transfer of the coating.